It is sometimes necessary to fabricate multiple conductor interconnect cables that have requirements to minimize voltage drops along the length of the conductors, yet have specific regions of the cable where flexibility is paramount. These two requirements can cause conflicts in the design of the cable.
Minimizing the voltage drop along the length of a specific conductor means fabricating the conductor with a greater cross-sectional area to reduce electrical resistance. This can be accomplished by increasing either the conductor width, thickness or both. Increasing the flexibility of a specific conductor means fabricating the conductor with a smaller cross-sectional area. This can be accomplished by decreasing either the conductor width, thickness or both.
If the conductor widths of a multiple conductor interconnect cable have already been reduced to the minimum practical value for manufacturing so as to maximize the number of conductors in the cable, then the only variable left to the designer in the trade off of electrical resistance and flexibility is to vary the conductor thicknesses along their length.
The method of producing electrical cable proposed in U.S. Pat. No. 5,274,195 (assigned to the assignee of the present application) discloses a chemical milling process for selectively reducing portions of electrically conductive outer laminae on both sides of a barrier layer which is resistant to the chemical milling process of the laminae prior to introduction of a supportive dielectric film. The manufacturing process which accomplishes this task involves the several steps of having to separately etch first one outer conductive lamina to produce a desired conductor thickness and define a desired conductor pattern, then etch the opposing outer conductive lamina to form the appropriate conductor pattern of multiple thicknesses and lastly to remove the barrier layer by a third etching process. The method of this prior art requires the support of the conductive pattern during the etching process by means other than the dielectric layer subsequently to be attached to the conductive pattern.
This method therefor requires the chemical milling process of the lamina to be undertaken individually and from both sides of the laminate thereby restricting the application of further lamina such as dielectric layers until the etching process on at least one side of the laminate is complete.